An antenna in its most basic form is a transducer designed to transmit or receive electromagnetic waves, thus converting electromagnetic radiation into electrical current, or vice versa. In particular, the electrical length of an optimum antenna element is related to the frequency of the signal that the antenna is designed to transmit or receive, i.e., the resonant frequency and electrical resonance of an antenna is related to the electrical length of the antenna element. The electrical length is usually the physical length of the element divided by its velocity factor (the ratio of the speed of wave propagation in the element to the speed of light). Typically, an antenna is tuned for a specific, resonant frequency, and is effective for a range of frequencies that are centered on the resonant frequency. Notably, however, other properties (e.g., the radiation pattern and impedance) of an antenna change with frequency, so the antenna may be optimized for an overall response at a desired frequency.
As is well understood in the art, the wavelength (λ) of an electromagnetic wave is calculated as the speed of light (C, roughly 3×10^8 m/s) divided by the frequency (f). Antennas are often designed with antenna elements that have an electrical length equal to a wavelength of interest, or a fraction of the wavelength (e.g., ½, ¼, ⅛, etc.) based on the properties of a transmitted or received signal, such as polarization and so forth. While an antenna will, for example, still transmit if the electrical length is not ideal for resonance, less of the power provided by the transmitter will actually become a useful output signal. Accordingly, the antenna will have reduced efficiency.
A dipole antenna is a well known type of antenna and consists of two element “halves” that are center fed. Generally, each half of the dipole antenna is roughly ¼ wavelength long, and with the antenna being fed from its center, the total electrical length is ½ wavelength long. Also, due to the configuration of a dipole antenna (that is, where the ends of the antenna correspond to anti-nodes and the center to nodes), the antenna resonates well. Dipole antennas are considered balance devices because they are symmetrical and work best when they are fed with a balanced current. In other words, the current is of equal size on both halves (e.g., and phase shifted 180 degrees). This is usually accomplished when the antenna is fed with an unbalanced feed, such as a coaxial cable, through a type of circuit or transformer called a balun (from BALanced and UNbalanced). Notably, the optimum size of a dipole antenna is slightly different than would be expected based on wavelength alone, due to the interaction of the balun and the antenna elements. However, the length is relatively close to the predicted length for optimum broadcast efficiency.
To achieve superior performance in many different scenarios, a type of cylindrical antenna known as a quadrifilar helix antenna (QHA) has been used for various types of communication, such as satellite systems. The quadrifilar helix antenna is generally composed of four identical antenna elements in the form of helixes wound, equally spaced, on a cylindrical surface. For transmitting, the helixes may be fed with signals equal in amplitude and 0, −90, −180, and −270 degrees in relative phase to produce circularly polarized electromagnetic radiation in the radio frequency or “RF” wavelengths. The QHA antenna provides a generally hemispherical radiation pattern (a signal polarized both vertically and horizontally). The QHA antennas are generally attractive for their small size and light weight, which makes them suitable for certain applications, such as for use with handheld handsets, also referred to as handhelds.
A stacked quadrifilar helix antenna, in particular, incorporates two QHA antennas, one located adjacent the other along the same cylindrical axis. For example, in an illustrative implementation, an upper antenna may serve the transmission of RF energy at one frequency and a lower antenna may be used to transmit or receive RF energy at another frequency. Often these frequencies may fall within the microwave frequency range, but the antenna may be designed for other frequencies as well. An example stacked QHA antenna and corresponding feed network is shown in U.S. Pat. No. 5,872,549, issued on Feb. 16, 1999 to Huynh et al. (“the '549 patent”), the content of which incorporated herein by reference in its entirety. In particular, the '549 patent describes an advanced form factor that uses a microstrip balun structure to reduce the size of the antenna's feed network.
While the '549 patent illustrates one manner to reduce the size of stacked quadrifilar helix antennas, there generally remains an ongoing desire to further reduce the size of an antenna's package for convenience and aesthetics, and to reduce manufacturing cost and complexity, while also maintaining acceptable levels of performance.